Help with LCD 4 digit display

[BobWa3]

I am trying to fix a 4 digit frequency display on a Dittel FSG 60M avionics transceiver. The radio is 25 years old and works perfectly, but last summer the frequency display began
to occasionally show all 8s instead of the tuned frequency. Gradually this got worse to the point where it now only shows.8's (although randomly it will occasionally show the correct frequency,
and if you drop the operating voltage on the radio below 5 volts the correct frequency will appear). I have a maintenance manual and a complete schematic for the radio.
The display board is relatively simple with very few components. I have measured the logic inputs on all of the driver chips and they are correct. All resistors on the board check out.
I don't have a good way to check the capacitors. The multivibrator chip is putting out a 5 volt square wave at oscillator pin(13) and at Q+ and Q- pins(10&11).
The Q outputs are half the frequency of the oscillator output. The LCD display is a mystery to me. I have no data sheet for it but the connection to the PCB is through elastomeric connectors.
I have attached a copy of the pertinent part of the schematic and pictures of the physical board.

Any help appreciated.

Unfortunately my photos cannot be uploaded for some reason

[edited by a moderator ... Please DONT do underlining like that again !
Also broke up the long string of text into something easier to read]

 

[Bluejets]

Underlined text makes for difficult reading....

 

[davenn]

Underlined text makes for difficult reading....

I have edited it

 

[Harald Kapp]

Possibly the contrast voltage is too high as indicated by a correct display at lower supply voltage. The chips are all standard components. They are likely all good. Can you provide a better resolved image of the connector on the lower right? This section:

Also check the supply voltages for correct voltage and low ripple. After 25 years the electrolytic capacitors are probably no longer doing their duty. Simply replacing these may help.

 

[BobWa3]

Sorry about the underlining, I thought the program selected it.
The radio is operated from a 12 volt battery supply. There are no electrolytic capacitors on the display board itself.
I have looked at the supply voltages coming into the board with a primitive oscilloscope and they appear clean. I have included a better picture of the connector to the circuit board. Connections are labeled:
20-Ground
19-Heating
18-no label
17-Dimmer
16-Ground
15-Blanking(digits flash when supply voltage drops below 11V)this works
14-14volt+
13-10volts+
1-12 are various frequency logics and they are correct
There is a lighting circuit which is not powered up in my application. There is also a heating circuit to warm the LCD display in cold temperatures.
How is the contrast set on an LCD? I really don't understand how to properly drive an LCD.
What is the reason for the square wave input to the LCD? Thanks for the reply.
Bob

 

[bertus]

Hello,

Have a look at this video:

Bertus

 

[Harald Kapp]

There is no explicit contrast voltage on this display, The LCD and the drivers are powered by 10 V (connector P2003, pin 13). CHeck the value of this voltage vs. GND. There is also a series resistor R2006 in the 10 V path. Check its value. Next to R2006 there is an electrolytic capacitor C2003. Check this or simply replace it. R2006 and C2003 form a low pass filter for the 10 V supply. If the filter doesn't work correctly, noise on the 10 V supply may have a negative influence on the operation of the display.

 

[BobWa3]

Hello,

Have a look at this video:

Bertus

Thank you for this link. I had looked all over the net for an explanation of how to drive an LCD but everything I found was about LEDs. The answer lies in this video but it will take me a while to fully digest it.


Great video. I believe the answer to my problem lies in fully understanding the information in this video. That may take a few more viewings, some additional research and some new measurement on the actual board. The 4047 chip on my board is putting out complementary 10 volt square waves from pin 10 and 11 which are fed to the common connection and the P connection on the LCD board which is also connected to a pin labeled "1". The first digit on the display is fixed at one. I have included a low resolution photo of the board. Thanks for your help. I may have some more questions after I have done a bit more reading.

 

[BobWa3]

There is no explicit contrast voltage on this display, The LCD and the drivers are powered by 10 V (connector P2003, pin 13). CHeck the value of this voltage vs. GND. There is also a series resistor R2006 in the 10 V path. Check its value. Next to R2006 there is an electrolytic capacitor C2003. Check this or simply replace it. R2006 and C2003 form a low pass filter for the 10 V supply. If the filter doesn't work correctly, noise on the 10 V supply may have a negative influence on the operation of the display.

I have checked resistor 2006 and it is precisely 100 ohms. The capacitor is not shorted but I haven't measured the actual value. I have included a picture of the actual board with a capacitor C2003 circled. Is this an electrolytic cap?

 

[BobWa3]

I have checked resistor 2006 and it is precisely 100 ohms. The capacitor is not shorted but I haven't measured the actual value. I have included a picture of the actual board with a capacitor C2003 circled. What type of capacitor would you recommend to replace this capacitor with?

 

[Martaine2005]

I have never seen one before.
Could it be a dipped tantalum?.

Martin

 

[Harald Kapp]

I agree, looks like a tantalum. Also supported by the comparatively low value of 1 µF only.

 

[Martaine2005]

@Harald Kapp can I ask how you get 1μF?
Brown, black, green and blue?
Does green and blue have no significance? Thanks.

Martin

 

[Harald Kapp]

Of course you may ask. Do you expect an answer?
I took the value from the schematic:

Brown, black, green and blue?

Where do you see blue?
Tantalum capacitor color markings are a bit different from e.g. resistors, see this example which incidentally is for a 1 µF capacitor.

 

[bertus]

Hello,

@Martaine2005 , Have a look at step 6 for the tantalium color code on the following page:
https://www.instructables.com/From-Resistors-to-ICs-Color-Codes/

I mentioned the site also in this thread here:
Electronics color code

bertus

 

[BobWa3]

Could someone explain the purpose of capacitor C2003 in this circuit, possible failure modes, and consequences of the failure modes? What measurements could I make to confirm the failure? Thanks again, the responses have been quite helpful.

 

[bertus]

Hello,

It looks like that C2003 is a decoupling capacitor.
One of the failure modes of a tantalium capacitor can be a short circuit.
Measure the voltage over the capacitor and see if it is around the 10 Volts that is coming in via the 100 Ohms resistor.

You can replace the capacitor with a tantalium capacitor again or an other electrolitic capacitor.

bertus

 

[Martaine2005]

I took the value from the schematic:

Hehe, sneaky!.
I never looked at the schematic. It is blurry when enlarged on my phone.
It looks like blue by the leads.
And I thought green was the multiplier and blue the tolerance.
Thanks for clearing that up.

Thanks @bertus , I have read that before.
Never sunk in about the extra colour through first and second colour. Pink in this case, 35v.

Martin

 

[Harald Kapp]

Could someone explain the purpose of capacitor C2003

See my post #7.

 

[BobWa3]

From post #7 "If the filter doesn't work correctly, noise on the 10 V supply may have a negative influence on the operation of the display." What would the noise look like, what would the negative effect be, how can I test for it?